Engine ??
 

Can you show the exhaust side?

Looks like an old Horner engine . Basically an early 90's Kawasaki engine modified for rc use. Most Horners were 60cc and 70cc .Later versions used Homelite cylinders IIRC .There were a few prototype 80cc versions out there .

Not sure what the engine is but the cylinders appear to be Sachs 2.4 ci / 42 cc same as used on Brison and Fox 2.4 engines. Easily identified by the offet exhaust bolts and distinct cylinder casting.

It looks exactly like the Horner case used on their other engines. Either Horner used the 2.4 cylinders onsome of the 80's or someone upgraded from the Homelite cylinders . Either way your probably in the 23 to 24 inch prop range with a 80cc . The crankshaft on the horners dont like big props or prop strikes

Not being familiar with the Horner engine, I looked up "Horner RC Engine Images" on Google and there are several good photos there. The versions there have different cylinders but the case and related parts are as shown above. The Sachs cylinders are more compact. Thanks CK1, I learned something new from this. Will probably forget what I learned by tomorrow but it is interesting today.